1. Field of the Invention
This invention relates generally to an apparatus and method for reducing nitric oxide (NOx) emissions and cooling the combustion liner for a can-annular gas turbine combustion system. Specifically, an apparatus and method for introducing the cooling air into the premix chamber of the combustion system that minimizes the use of compressor discharge air for cooling the combustion liner as well as for improving the mixing of fuel and air prior to the combustion process.
2. Description of Related Art
Combustion liners are commonly used in the combustion section for most gas turbine engines. The combustion section is located between the compressor and turbine, and depending upon the application, the combustion section may not be located along the centerline of the engine, but may be located around the centerline or even perpendicular to the engine orientation. Combustor liners serve to protect the combustor casing and surrounding engine from the extremely high operating temperatures by containing the chemical reaction that occurs between the fuel and air.
Recent government emission regulations have become of great concern to both gas turbine manufacturers and operators. Of specific concern is nitric oxide (NOx) due to its contribution to air pollution. While NOx emissions are of some concern to aircraft engines, greater concerns include engine weight, performance, safety and fuel efficiency. While these concerns are shared by the industrial gas turbine engine market, NOx emissions rank as one of the greatest concerns. Utility sites have governmental permits that allow specific amounts of emissions each year. Lower emission rates, especially NOx, allow engines to run longer hours and hence generate more revenue.
It is well known that NOx formation is a function of flame temperature, air inlet temperature, residence time, and equivalence ratio. Nitric oxide emissions have been found to be lower for lower flame temperature, lower air inlet temperature, shorter residence time, and lower equivalence ratio, or a leaner fuel mixture. Lower flame temperatures and lower equivalence ratios can be accomplished by increasing the amount of air used in the combustion process, for a given amount of fuel. Further reductions in emissions can be accomplished by improving the utilization of the cooling air.
The present invention is used in a dry, low NOx gas turbine engine, which is typically used to drive electrical generators. Each combustor includes an upstream premix fuel/air chamber and a downstream, or secondary, combustion chamber, separated by a venturi having a narrow throat constriction. The present invention is concerned with improving the mixing of fuel and air in the premix zone as well as the cooling of the combustion liner to further reduce nitric oxide emissions.
Prior methods of cooling combustion liners vary extensively. U.S. Pat. No. 4,292,801 and U.S. Pat. No. 5,127,221 describe louver film cooling and transpiration cooling, respectively, for similar dual-stage, dual-mode combustors. Backside impingement cooling is described in U.S. Pat. No. 5,117,636. Though these methods of cooling have proven adequate throughout the engine operating cycle, enhancements can be made to further reduce pollutants from the combustor, while improving cooling effectiveness.
Over the years, some annular gas turbine combustor designers have incorporated angled film cooling holes, specifically for improving cooling efficiency. Typically, annular combustors are used for aircraft applications where small size and reduced weight are important design factors. Angled film cooling holes improve cooling efficiency by increasing the amount of internal surface area that is available for heat removal. For example, a hole drilled at 20 degrees to the liner wall has nearly three times as much surface area as a hole drilled normal to the liner surface. In addition, angled film cooling holes provide a jet of air to form a better film along the liner surface. In order to accomplish this improved cooling, thicker liner walls are typically required, which further increase hole surface area, hence an increase in liner weight. Examples of annular aircraft combustors utilizing this cooling technique are discussed in U.S. Pat. No. 5,233,828; U.S. Pat. No. 5,181,379; U.S. Pat. No. 5,279,127; and U.S. Pat. No. 5,261,223. This technique is also used in an annular liner dome plate as described in U.S. Pat. No. 5,307,637, and to provide differential cooling to accommodate hot spots on annular combustor liner surfaces, as discussed in U.S. Pat. No. 5,241,827.
Of greater importance to reduce NOx emission than the improved cooling is the improved mixing of the air with fuel for combustion. When cooling performance is improved, less air is typically required for cooling and more can be dedicated to fuel/air mixing. More air into the combustion process will lower fuel to air ratios and hence equivalence ratio as well as lower flame temperature, which, as explained earlier, are two key drivers of NOx emissions. The increased air for the combustion process can be delivered through the front end of the combustor with the fuel or through the cooling holes as part of the jet. The jet of air would then provide the cooling film for the liner surface as well as a jet of air to mix with the fuel prior to combustion. This increase in mixing performance can be improved further by angling the holes in the circumferential direction to induce a swirl within the combustor.
The present invention provides for improved combustor cooling while enhancing fuel/air mixture in the combustor for a dual-stage, dual-mode low NOx combustor with a dedicated premix chamber.
An improved apparatus and method for mixing fuel and air, while at the same time cooling a gas turbine combustion liner in a can-annular low NOx gas turbine engine that includes a gas turbine combustor having a premixing chamber, a secondary combustion chamber with a venturi, described as a dual-mode, dual-stage combustor. Each gas turbine engine typically has a plurality of combustors.
In accordance with the present invention, each can-annular combustion liner is substantially cylindrical and includes an array of multiple film cooling apertures and dilution cooling apertures disposed in a predetermined array and direction of air flow, resulting in improved cooling performance on the combustion liner, while at the same time providing improved fuel and air mixture in the combustor.
The array of multiple film holes in each can-annular combustion liner includes angling each of the film cooling holes or apertures, both in an axial direction and a circumferential direction. The directionality of air flowing through the angled holes provides for a predetermined flow pattern within the combustion liner that aids in fuel/air mixing. The combustion liner apertures and holes are produced by drilling holes through the combustion liner at a predetermined angular slant in the direction of combustion flow, cold side to hot side. The predetermined strategic slanted or angled aperture is not perpendicular to the combustor wall. A predetermined angle that is in two directions, both axially and circumferentially, is selected to increase the amount of surface area of the combustion liner that is being cooled, while at the same time providing directionality of flow that greatly enhances the mixing of fuel and air. The slanted holes are drilled at a circumferential angle that is preferably in the direction of combustor swirl from the premix chamber. The diameter of each of the holes and the spacing of the holes from each other is sized to maximize the cooling effectiveness of the hole pattern, improve fuel/air mixing, while at the same time not sacrificing the structural integrity of the combustion liner.
The apparatus described in this invention may include the combustor venturi section and air cooling flow as described in U.S. patent application Ser. No. 09/605,765 entitled xe2x80x9cCombustion Chamber/Venturi Cooling For A Low NOx Emission Combustorxe2x80x9d assigned to the same assignee as the present invention. The combustion liner also contains a dome section, which engages the fuel nozzles and provides another means for introducing air into the combustion process.
It is an object of the present invention to reduce the nitric oxide (NOx) emissions in a gas turbine combustion system by improving fuel/air mixing and lowering flame temperature.
It is another object of the present invention to provide a can-annular low emissions combustor system having combustion liners with apertures or holes for cooling and fuel/air mixing that are slanted axially and circumferentially.
It is yet another object of the present invention to incorporate an improved venturi section that utilizes its cooling air for the combustion process, further reducing polluting emissions.
In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.